This invention relates to a method for making lithographic printing plates directly from a block copy, that is, a method for the direct making of lithographic printing plates. More particularly, it relates to a method for the direct making of multicolor printing plates by using a panchromatic photosensitive material of the lithographic type and directly photographing an original bearing a continuous tone image and a line image.
At present, a large variety of photosensitive materials of the lithographic type are known and many of them are in actual use. As typical examples, there may be mentioned presensitized plates employing a diazonium compound; electrophotographic materials employing zinc oxide or an organic photoconductor; and silver salt photographic materials employing a silver halide emulsion. Especially, the sensitive materials having a high sensitivity such as the latter two of the above sensitive materials are suitable for use in direct plate making and have become remarkably popular, because they permit convenient and rapid plate making in an automatic plate making machine starting from the photographing of an original to be printed and throughout subsequent processing. In particular, those sensitive materials for direct plate making which employ silver salts as the sensitive constituents are useful because of their sensitivity, contrast, and resolving power which are very high.
There has been proposed a process of multicolor printing which takes advantage of the method of direct printing plate making. In this process, a panchromatic sensitive material of the lithographic type is brought into close contact with a contact screen and is exposed to a block copy bearing continuous tone images, e.g. a color photograph, and line images, e.g. letters or lines using color separation filters to make a cyan plate, a magenta plate, a yellow plate and a black plate.
For performing color printing with known presensitized plates, an image of photograph (called "continuous tone image" hereinafter) and an image of letters or lines (called "line image" hereinafter) are separately printed on a lith film to produce intermediate originals and these lith films are contact-printed on the presensitized plates in conformity with the composition.
However, the direct plate making method does not use intermediate originals such as lith films and hence must use a complete block copy original having continuous tone images and line images constructed in conformity with the desired composition. Therefore, the method requires exposure to a continuous tone image using contact screen and exposure to a line image without contact screen. For separate exposures to these two images, while one image which should not be subjected to exposure is covered with a mask, another image is subjected to exposure. However, in this case there are problems that a half tone dot image is formed in the line image area and half tone dots in the continuous tone image area become small or disappear. In most cases, the block copy bears the line images which are to be printed in an intended color by using at least one of the cyan, magenta, and yellow plates and other line images which are to be printed in black by using generally a black plate. This case presents problems such that in making a cyan, magenta, or yellow plate, the exposure should be performed so that a halftone dot image may not be formed in the areas of a sensitive material where the line image in color is to be formed, while in making a black plate the exposure should be performed so that any image may not be formed in the areas of line images and the halftone dots in the areas of continuous tone images may not be injured.